Sheet feeder and image forming apparatus incorporating the same

ABSTRACT

A sheet feeder includes a stacker to stack sheets and a separation and conveyance device to separate one sheet from the sheets fed from the stacker and convey the sheets one by one. The separation and conveyance device includes a feed roller that rotates in a direction to feed the sheet and a separation roller to press against the feed roller via the sheet. The separation roller has an apparent hardness different from an apparent hardness of the feed roller and is configured to form an indentation in the feed roller.

CROSS-REFERENCE TO RELATED APPLICATION

This patent application is based on and claims priority pursuant to 35 U.S.C. § 119 to Japanese Patent Application No. 2017-224500, filed on Nov. 22, 2017 in the Japanese Patent Office, the entire disclosure of which is hereby incorporated by reference herein.

BACKGROUND Technical Field

This disclosure relates to a sheet feeder and an image forming apparatus including the sheet feeder.

Description of the Related Art

Image forming apparatuses such as copiers and printers include a separation and conveyance device including a friction reverse roller (FRR) separator and a friction roller (FR) separator to separate and feed sheets of recording media one by one from a stack of sheets stacked on a sheet feeding tray. The FRR separator separates the sheet from the stack of sheets at a nip between a feed roller and a separation roller that is a friction reverse roller configured to press against the feed roller and that is subjected to a predetermined torque in a direction opposite to a feeding direction from a torque limiter. The FR separator separates the sheet from the stack of sheets at a nip between the feed roller and the separation roller that is a friction roller configured to press against the feed roller and that is supported by a rotational shaft via the torque limiter.

SUMMARY

This specification describes an improved sheet feeder that includes a stacker to stack sheets of recording media and a separation and conveyance device to separate one sheet from rest of the sheets fed from the stacker and convey the sheets one by one. The separation and conveyance device includes a feed roller that rotates in a direction to feed the sheet and a separation roller to press against the feed roller via the sheet. The separation roller has an apparent hardness different from an apparent hardness of the feed roller and is configured to form an indentation in the feed roller.

This specification further describes an improved sheet feeder that includes a stacker to stack sheets and a separation and conveyance device to separate one sheet from the sheets fed from the stacker and convey the sheets one by one. The separation and conveyance device includes a feed roller that rotates in a direction to feed the sheet and a separation roller to press against the feed roller via the sheet. The separation roller has an apparent hardness harder than an apparent hardness of the feed roller.

BRIEF DESCRIPTION OF THE DRAWINGS

The aforementioned and other aspects, features, and advantages of the present disclosure would be better understood by reference to the following detailed description when considered in connection with the accompanying drawings, wherein:

FIG. 1 is a schematic diagram of an image forming apparatus according to an embodiment of the present disclosure;

FIG. 2A is a perspective view illustrating a sheet feeder according to the embodiment of the present disclosure;

FIG. 2B is a side view illustrating a sheet feeder according to the embodiment of the present disclosure;

FIG. 3A is a side view illustrating a pair of rollers not pressed against each other and used in the sheet feeder according to the first embodiment of the present disclosure;

FIG. 3B is a side view illustrating a pair of rollers pressed against each other and used in the sheet feeder according to the first embodiment of the present disclosure;

FIG. 4A is a perspective view illustrating an assembled feed roller of the pair of rollers used in the sheet feeder according to a second embodiment of the present disclosure;

FIG. 4B is an exploded perspective view illustrating the feed roller of the pair of rollers used in the sheet feeder according to the second embodiment of the present disclosure;

FIG. 4C is a side view illustrating the pair of rollers including the feed roller used in the sheet feeder according to the second embodiment of the present disclosure;

FIG. 4D is a side view illustrating the pair of rollers including the feed roller used in the sheet feeder according to the second embodiment of the present disclosure;

FIG. 5A is a perspective view illustrating an assembled feed roller of the pair of rollers used in the sheet feeder according to a third embodiment of the present disclosure;

FIG. 5B is an exploded perspective view illustrating the feed roller of the pair of rollers used in the sheet feeder according to the third embodiment of the present disclosure;

FIG. 5C is a side view illustrating the pair of rollers including the feed roller used in the sheet feeder according to the third embodiment of the present disclosure;

FIG. 5D is a side view illustrating the pair of rollers including the feed roller used in the sheet feeder according to the third embodiment of the present disclosure;

FIG. 6A is a perspective view illustrating an assembled feed roller of the pair of rollers used in the sheet feeder according to a fourth embodiment of the present disclosure;

FIG. 6B is an exploded perspective view illustrating the feed roller of the pair of rollers used in the sheet feeder according to the fourth embodiment of the present disclosure;

FIG. 6C is a side view illustrating the pair of rollers including the feed roller used in the sheet feeder according to the fourth embodiment of the present disclosure; and

FIG. 6D is a side view illustrating the pair of rollers including the feed roller used in the sheet feeder according to the fourth embodiment of the present disclosure.

The accompanying drawings are intended to depict embodiments of the present disclosure and should not be interpreted to limit the scope thereof. The accompanying drawings are not to be considered as drawn to scale unless explicitly noted.

DETAILED DESCRIPTION OF EMBODIMENTS

In describing embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this specification is not intended to be limited to the specific terminology so selected and it is to be understood that each specific element includes all technical equivalents that have a similar function, operate in a similar manner, and achieve a similar result.

Although the embodiments are described with technical limitations with reference to the attached drawings, such description is not intended to limit the scope of the disclosure and all of the components or elements described in the embodiments of this disclosure are not necessarily indispensable.

Referring now to the drawings, embodiments of the present disclosure are described below. In the drawings illustrating the following embodiments, the same reference codes are allocated to elements (members or components) having the same function or shape and redundant descriptions thereof are omitted below.

With reference to drawings, descriptions are given of a sheet feeder according to embodiments of the present disclosure and an image forming apparatus such as a laser printer including the sheet feeder. It is to be noted that identical reference numerals are assigned to identical components or equivalents and description of those components is simplified or omitted. Further, size (dimension), material, shape, and relative positions used to describe each of the components and units are examples, and the scope of this disclosure is not limited thereto unless otherwise specified.

In the following embodiments, a sheet member is described as a sheet. However, the sheet is not limited to paper. The sheet may be not only the sheet made of paper but also an OHP sheet, a cloth, a metal sheet, a plastic film, or a prepreg sheet impregnated with resin on carbon fiber.

The sheet includes all of what is called a medium, which a developer or ink can adhere, such as a recording medium and a recording sheet. The term “sheet” is not limited to a plain paper but also includes thick paper, post card, envelope, thin paper, coated paper, art paper, and tracing paper.

Further, it is to be noted that the term “image formation” indicates an action for providing (i.e., printing) not only an image having meanings such as texts and figures on a recording medium but also an image having no meaning such as patterns on a recording medium.

Configuration of Image Forming Apparatus

FIG. 1 is a schematic diagram illustrating a configuration of an image forming apparatus such as a color laser printer including a sheet feeder of an embodiment of the present disclosure. As illustrated in FIG. 1, the image forming apparatus 100 includes four process units 1K, 1Y, 1M, and 1C serving as image forming devices. Suffixes, which are K, Y, M, and C, are used to indicate respective colors of toners (e.g., black, yellow, magenta, and cyan toners) for the process units. The process units 1K, 1Y, 1M, and 1C have substantially the same configuration except for containing different color toners of black (K), yellow (Y), magenta (M), and cyan (C) corresponding to color separation components of a color image.

The process units 1K, 1Y, 1M, and 1C have the same structure, differing only in the colors of toners in the toner bottles 6K, 6Y, 6M, and 6C. Therefore, the process unit 1K is described as a representative, and the descriptions of other process units 1Y, 1M, 1C are omitted.

The process unit 1K includes an image bearer 2K such as a photoconductor drum, a drum cleaning device 3K, and a discharger. Additionally, the process unit 1K includes a charger 4K to uniformly charge a surface of the image bearer 2K, a developing device 5K to visualize an electrostatic latent image formed on the image bearer 2K, and other devices. The process unit 1K, which is removable, is installed in the image forming apparatus 100 so that consumables are replaceable at a time.

The image forming apparatus 100 further includes an exposure device 7 disposed above the process units 1K, 1Y, 1M, and 1C. The exposure device 7 performs scanning and writing based on image data, that is, emits laser lights L from laser diodes disposed therein based on the image data.

A transfer device 15 is disposed beneath the process units 1K, 1Y, 1M, and 1C in the configuration illustrated in FIG. 1. Primary-transfer rollers 19K, 19Y, 19M, and 19C are disposed in contact with an intermediate transfer belt 16 and to face the image bearers 2K, 2Y, 2M, and 2C, respectively.

The intermediate transfer belt 16 is looped around the primary-transfer rollers 19K, 19Y, 19M, and 19C, a driving roller 18, and a driven roller 17 and rotates. A secondary-transfer roller 20 is disposed facing the driving roller 18 and contacting the intermediate transfer belt 16. It is to be noted that, when the photoconductor drums 2 are called primary image bearers, the intermediate transfer belt 16 is called a secondary image bearer to bear a synthesized image made from images formed on the respective photoconductor drums 2.

A belt cleaning device 21 is disposed downstream from the secondary-transfer roller 20 in a direction of rotation of the intermediate transfer belt 16. A cleaning backup roller is disposed opposite to the belt cleaning device 21 with the intermediate transfer belt 16 sandwiched therebetween.

The image forming apparatus 100 further includes a sheet feeding tray 50 including a sheet stacker 260 in which sheets are stacked. The sheet feeding tray is disposed at a lower part of the image forming apparatus 100 and can accommodate a stack of the sheets P. The sheet feeding tray 50 is detachably attachable to an apparatus body of the image forming apparatus 100 for sheet supply, for example. A sheet feed roller 60 serving as a sheet feeding device and a pair of rollers 210 are disposed above the sheet feeding tray 50 in the image forming apparatus 100 as illustrated in FIG. 1 and feeds the top sheet P from the stack of sheets placed in the sheet feeding tray 50 from the sheet feeding tray 50 to a sheet feeding path 32.

A pair of registration rollers 250 is disposed upstream from the secondary-transfer roller 20 in a sheet conveying direction of the sheet P. The pair of registration rollers 250 includes two rollers that form a registration nip region between the two rollers. The sheet P that is fed from the sheet feeding tray 50 abuts against the registration nip region and stops temporarily. By temporarily stopping the sheet P at the registration nip region of the pair of registration rollers 250, the sheet P sags at the leading end of the sheet.

A registration sensor 31 is disposed upstream from the pair of registration rollers 250 in the sheet conveying direction and near the pair of registration rollers 250. The registration sensor 31 detects passage of the leading edge of the sheet P. When a predetermined time has elapsed after the registration sensor 31 detects the passage of the leading edge of the sheet P, the sheet P contacts the registration nip region and stops temporarily.

As illustrated in FIGS. 2A and 2B, the sheet feed roller 60 and the pair of rollers 210 are disposed in a unitized sheet feeder 200. As illustrated in FIG. 2B, the sheet feeder 200 includes a conveyance roller 240 on the downstream side of the sheet feeder 200 in the sheet conveying direction. The sheet P is conveyed from the pair of rollers 210 to right side in FIG. 2B, and the conveyance roller 240 conveys the sheet P upward. As illustrated in FIG. 1, the conveyance roller 240 conveys the sheet P toward the upper pair of registration rollers 250.

The pair of rollers 210 serving as a separation and conveyance device includes a pair of upper and lower rollers. The pair of rollers 210 may be an FRR separator or an FR separator. The FRR separator separates the sheet from the stack of sheets at a nip between a feed roller and a separation roller that is a friction reverse roller that presses against the feed roller and is subjected to a predetermined torque from a drive shaft in a direction opposite to a feeding direction via a torque limiter. The FR separator separates the sheet from the stack of sheets at a nip between the feed roller and the separation roller that is a friction roller that presses against the feed roller and is supported by a rotational shaft via the torque limiter.

In the present embodiment, the pair of rollers 210 is configured as the FRR separator. That is, the pair of rollers 210 includes the upper feed roller 220 to convey the sheet P in the image forming apparatus 100 and the lower separation roller 230 that is given a drive force from the drive shaft in a direction opposite to the feeding direction of the feed roller 220 via the torque limiter. The feed roller 220 is supported by the rotational shaft 270, and the separation roller 230 is supported by the rotational shaft 280.

The separation roller 230 is biased by a biasing member such as a spring toward the feed roller 220. The sheet feed roller 60 receives the drive force of the feed roller 220 via a clutch and rotates to the left (counter-clockwise (CCW)) in FIG. 2B.

The pair of registration rollers 250 sends the sheet P, which contacts the pair of registration rollers 250 and sags, to a secondary-transfer nip formed between the secondary-transfer roller 20 and the driving roller 18 when a toner image formed on the intermediate transfer belt 16 is appropriately transferred onto the sheet P. A secondary-transfer bias applied in the secondary-transfer nip electrostatically transfers the toner image formed on the intermediate transfer belt 16 onto a designated position on the sheet P with high accuracy.

Then, the sheet P is transported through a post-transfer sheet conveyance path 33 disposed above the secondary-transfer nip between the secondary-transfer roller 20 and the driving roller 18 in FIG. 1. A fixing device 34 is disposed in a vicinity of an upper end of the post-transfer sheet conveyance path 33. The fixing device 34 includes a fixing roller 34 a in which a heat source such as a halogen lamp is provided and a pressure roller 34 b to rotate in contact with the fixing roller 34 a at a predetermined pressure. The fixing device 34 may be a fixing device using an endless looped belt or a fixing device using an IH heating method.

A post-fixing conveyance path 35 is disposed above the fixing device 34. The post-fixing conveyance path 35 branches at a downstream end thereof into two paths, which are a sheet ejection path 36 and a reverse conveyance path 41. A route switch 42 is provided in a branching portion thereof. The route switch 42 pivots about a pivot shaft 42 a. A pair of sheet ejection rollers 37 is disposed at an opening end of the sheet ejection path 36.

The reverse conveyance path 41 converges with the sheet feeding path 32 at an end opposite the branching portion. Additionally, a pair of reverse conveyance rollers 43 is disposed midway in the reverse conveyance path 41. An upper face of the image forming apparatus 100 is recessed to an inner side of the image forming apparatus 100 and serves as an output tray 44.

A toner container 10 serving as a powder container is provided between the transfer device 15 and the sheet feeding tray 50. The toner container 10, which is removable, is installed in the apparatus body of the image forming apparatus 100.

In the present embodiment, for the purpose of sheet conveyance, a predetermined length is secured from the sheet feed roller 60 to the secondary-transfer roller 20. The toner container 10 is disposed in a dead space caused by that distance to keep the entire image forming apparatus compact.

A transfer cover 8 is provided above the sheet feeding tray 50 and on a front side to which the sheet feeding tray 50 is pulled out. The transfer cover 8 is openable to check an interior of the image forming apparatus 100. To the transfer cover 8, a feeding roller 45 and a bypass tray 46 for bypass feeding are provided.

It is to be noted that the image forming apparatus 100 according to the present embodiment is not limited to the laser printer. That is, the image forming apparatus can be a copier, a facsimile machine, a printer, a plotter, and a multifunction peripheral having at least two of copying, printing, facsimile transmission, plotting, and scanning capabilities; or an inkjet recording device.

Operation of Image Forming Apparatus

Referring to FIG. 1, operation of the image forming apparatus 100 according to the present embodiment is described below. Initially, single-side printing is described. Referring to FIG. 1, the sheet feed roller 60 rotates in response to a sheet feeding signal from a controller of the image forming apparatus 100. Then, the sheet feed roller 60 separates the top sheet P from the stack of sheets placed in the sheet feeding tray 50 and forwards the sheet P to the sheet feeding path 32.

When the leading edge of the sheet P fed by the sheet feed roller 60 and the pair of rollers 210 reaches the nip between the pair of registration rollers 250, the sheet P is slackened and kept standby. Then, sheet conveyance is suspended for the timing of transfer of the toner image from the intermediate transfer belt 16, and skew of the sheet P at the leading edge is corrected.

In the case of bypass feeding, a stack of sheets placed on the bypass tray 46 are fed one by one from the top by the feeding roller 45 and transported through a part of the reverse conveyance path 41 to the pair of registration rollers 250. Operations thereafter are similar to those of sheet feeding from the sheet feeding tray 50.

As to image formation, operations of the process unit 1K are described as a representative. First, the charger 4K uniformly charges a surface of the image bearer 2K to high potential. The exposure device 7 emits a laser beam L onto the surface of the image bearer 2K according to image data.

On the surface of the image bearer 2K, a portion irradiated with the laser beam is reduced in potential, thus forming an electrostatic latent image. The developing device 5K includes a developer bearer to bear the developer including toner and supplies black toner from the toner bottle 6K onto the portion of the image bearer 2K bearing the electrostatic latent image via the developer bearer to develop the electrostatic latent image into a black toner image. The toner image is then transferred from the image bearer 2K onto the intermediate transfer belt 16 (i.e., a primary-transfer process).

The drum cleaning device 3K removes toner remaining on the surface of the image bearer 2K after the primary-transfer process. The toner thus removed is transported by a collected-toner conveyor to a collected-toner container in the process unit 1K. The discharger removes electricity remaining on the surface of the image bearer 2K cleaned by the drum cleaning device 3K.

Toner images are formed on the respective photoconductor drum 2 in other process units 1 as well, and respective color toners are superimposed one on another on the intermediate transfer belt 16.

The superimposed color toner images transferred onto the intermediate transfer belt 16 reaches the secondary-transfer nip between the secondary-transfer roller 20 and the driving roller 18. The pair of registration rollers 250 rotates and sandwiches the sheet P contacting the pair of registration rollers 250, and sends the sheet P to the secondary-transfer nip formed by the secondary-transfer roller 20 when the superimposed color toner images on the intermediate transfer belt 16 are appropriately transferred onto the sheet P. In this way, the toner image on the intermediate transfer belt 16 is transferred onto the sheet P sent by the pair of registration rollers 250.

The sheet P on which the toner image is transferred is transported through the post-transfer sheet conveyance path 33 to the fixing device 34. The sheet P transported to the fixing device 34 is sandwiched by the fixing roller 34 a and the pressure roller 34 b, and pressure and heat fix the toner image on the sheet P. The fixing device 34 sends out the sheet P carrying the fixed toner image to the post-fixing conveyance path 35.

When the fixing device 34 sends out the sheet P, the route switch 42 is at a position indicated by solid lines to open an upper end portion of the post-fixing conveyance path 35 in FIG. 1. After the sheet P is sent from the fixing device 34, the sheet P is sent to the sheet ejection path 36 via the post-fixing conveyance path 35. The sheet P sent to the sheet ejection path 36 is sandwiched by the pair of sheet ejection rollers 37, and rotation of the pair of sheet ejection rollers 37 ejects the sheet P to the output tray 44. By performing this operation, a series of the single-side printing operations is completed.

Next, duplex printing is described. Similar to single-side printing, the fixing device 34 sends out the sheet P to the sheet ejection path 36. When performing duplex printing, the pair of sheet ejection rollers 37 rotates to convey the sheet P so that part of the sheet P is exposed to an outside of the image forming apparatus 100.

When a trailing end of the sheet P passes by the branching portion to the sheet ejection path 36, the route switch 42 pivots about the pivot shaft 42 a as indicated by broken lines in FIG. 1. Thus, the upper end of the post-fixing conveyance path 35 is closed. When the upper end of the post-fixing conveyance path 35 is closed, nearly simultaneously, the pair of sheet ejection rollers 37 rotates in reverse to transport the sheet P to an inner side of the image forming apparatus 100, to the reverse conveyance path 41.

The sheet P fed to the reverse conveyance path 41 travels through the pair of reverse conveyance rollers 43 and reaches the pair of registration rollers 250. The pair of registration rollers 250 forward the sheet P to the secondary-transfer nip, timed to coincide with transfer of the toner image from the intermediate transfer belt 16.

While the sheet P passes through the secondary-transfer nip, the secondary-transfer roller 20 and the driving roller 18 transfer the toner image onto a second side (backside) of the sheet P. Then, the sheet P is transported through the post-transfer sheet conveyance path 33 to the fixing device 34.

In the fixing device 34, the sheet P is sandwiched by the fixing roller 34 a and the pressure roller 34 b to fix the unfixed toner image formed on the backside of the sheet P to the sheet S by application of heat and pressure. The fixing device 34 sends out the sheet P carrying the fixed toner image on both sides to the post-fixing conveyance path 35.

When the fixing device 34 sends out the sheet P, the route switch 42 is at a position indicated by solid lines to open an upper end portion of the post-fixing conveyance path 35 in FIG. 1. After traveling from the fixing device 34, the sheet P is conveyed to the sheet ejection path 36 via the post-fixing conveyance path 35. The sheet P sent to the sheet ejection path 36 is sandwiched by the pair of sheet ejection rollers 37, and rotation of the pair of sheet ejection rollers 37 ejects the sheet P to the output tray 44. By performing this operation, a series of the duplex printing operations is completed.

After the toner image on the intermediate transfer belt 16 is transferred onto the sheet P, there is toner remaining on the intermediate transfer belt 16. The belt cleaning device 21 removes the remaining toner from the intermediate transfer belt 16. The toner thus removed is transported by a waste-toner conveyor to the toner container 10.

Roller Pair

With reference to FIGS. 3A to 6B, a description is provided of rollers used for the pair of rollers 210 in the sheet feeder 200 according to the present embodiment of the present disclosure. FIGS. 3A to 6B illustrate different first to fourth embodiments, respectively.

In all the embodiments, the feed roller 220 and the separation roller 230 in the pair of rollers 210 include rubber elastic layers 222 and 232, respectively. Rubber materials of the elastic layers 222 and 232 of the feed roller 220 and the separation roller 230 are the same to prevent swelling, a description of which is deferred.

The rubber materials are polymer materials having rubber elasticity at room temperature or materials thereof, for example, natural rubber (NR), isoprene rubber (IR), styrene butadiene rubber (SBR), ethylene-propylene-diene monomer (EPDM), chloroprene rubber (CR), acrylonitrile-butadiene rubber (NBR), urethane rubber (U), epichlorohydrin rubber (CO, ECO), silicone rubber (Q), and fluorine rubber (FKM).

Further, in all the embodiments, apparent hardness of the separation roller 230 is made harder than the apparent hardness of the feed roller 220 to improve forward rotation performance and separation performance of the separation roller 230. Any difference in geometrical structure between the feed roller 220 and the separation roller 230 gives magnitude difference of the apparent hardness. Therefore, the forward rotation performance and the separation performance of the separation roller 230 can be improved without being affected by environmental factors such as temperature and humidity and chronological change of wear amounts of rollers.

Types of “hardness” are “standard hardness” and “apparent hardness”. Standard hardness is measured based on standard measurement methods in which the size and shape of a test piece satisfy conditions defined by the standard measurement methods. When the apparent hardness is measured based on the standard measurement method, the size and shape of the test piece do not satisfy the conditions defined by the standard measurement method (see JIS K 6253-2). The apparent hardness is hardness that is brought about by the structure of a part. The apparent hardness may be relatively measured by a deformation amount when the part is pushed by a predetermined pressure based on JIS K 6253-2.

In the present embodiment, the apparent hardness is made different between the feed roller 220 and the separation roller 230 so that the separation roller dents the feed roller 220 and forms a nip. Rubber thicknesses in the elastic layers 222 and 232 change the apparent hardness of the feed roller 220 and the separation roller 230, respectively.

Following first to third embodiments illustrate different methods to change the rubber thickness. In the fourth embodiment, the apparent hardness is made different by the concavo-convex portion of the outer peripheral surface of the hub 221 without changing the rubber thickness.

First Embodiment

FIGS. 3A and 3B illustrate a pair of rollers 210 used in the sheet feeder according to the first embodiment. In FIG. 3A, the separation roller 230 does not push the feed roller 220. In FIG. 3B, the separation roller 230 pushes the feed roller 220. Since elastic force of a spring or the like push the lower separation roller 230 toward the upper feed roller 220, the separation roller 230 presses against the feed roller 220 via the sheet P. Therefore, the nip is formed between the feed roller 220 and the separation roller 230 with a predetermined contact pressure.

The feed roller 220 and the separation roller 230 include hubs 221 and 231 that are hard cylinders supported on rotational shafts of the feed roller 220 and the separation roller 230 and elastic layers 222 and 232 that are cylinders fitted to the outer peripheral surfaces of the hubs 221 and 231, respectively. The hubs 221 and 231 include inner cylinders 221 a and 231 a fitted to the rotational shafts of the feed roller 220 and the separation roller 230, external cylinders 221 b and 231 b, and a plurality of spokes 221 c and 231 c that connect the external cylinders 221 b and 231 b and the inner cylinders 221 a and 231 a in the radial direction, respectively. The rotational shafts of the feed roller 220 and the separation roller 230 are fitted into the holes 221 d and 231 d of the inner cylinders 221 a and 231 a, respectively.

The feed roller 220 and the separation roller 230 have substantially the same diameter, thus facilitating separation and conveyance of the sheet between the two rollers. Preferably, the feed roller 220 and the separation roller 230 have the same outer diameter, but it is not necessary for the feed roller 220 and the separation roller 230 to be completely the same outer diameter. One of the outer diameters of the feed roller 220 and the separation roller 230 may be within 2 times, 1.5 times, or 1.2 times of the other.

The thickness of the elastic layer 222 in the radial direction of the feed roller 220 is larger than the thickness of the elastic layer 232 in the radial direction of the separation roller 230. In the first embodiment, the thicknesses of the elastic layers 222 and 232 in each radial direction which are different as described above make the apparent hardness of the separation roller 230 harder than the apparent hardness of the feed roller 220. The above-described difference of the apparent hardness makes a form of the nip N between the feed roller 220 and the separation roller 230 to be a shape concave to the feed roller 220, in other words, a shape formed by the separation roller 230 that dents the feed roller 220.

The shape and diameter of the separation roller 230 in FIG. 3B are almost the same as the shape and diameter of the separation roller 230 in FIG. 3A. Since the elastic layer 232 of the separation roller 230 has elasticity similar to the elastic layer 222 of the feed roller 220, actually, the shape and diameter of the separation roller 230 slightly deforms. However, appropriately and largely setting the difference in thickness in the radial direction of the elastic layers 222 and 232, for example, 2:1, makes it possible to substantially ignore an effect that the slight deformation of the separation roller 230 has on the nip shape in which the feed roller 220 becomes concave.

The upward convex nip shape as illustrated in FIG. 3B increases hysteresis friction force at the nip N. The hysteresis friction force is generated on the surface of the roller by resilience of deformed rubber.

Further, the upward convex nip shape reduces a radial deformation amount (that is, an indentation amount) of the separation roller 230. This ensures a long distance from the outer circumferential surface of the elastic layer 232 to the rotation center of the hub 231 and reduces the rotational force to rotate the separation roller 230 in the forward direction.

Under high pressure at the upward convex nip N having the long distance to the rotation center of the hub 231, the rotational force of the feed roller 220 works the separation roller 230, which improves the forward rotation performance of the separation roller 230. When one sheet is sandwiched in the nip N and sent from the nip N, the high forward rotation performance and the increase in the hysteresis friction force attains smooth feeding with no slip with the sheet. When the sheet P containing much calcium carbonate, less slip of the separation roller 230 reduces drop of the calcium carbonate from the sheet P.

When a plurality of sheets is sandwiched in the nip N, the increase in the hysteresis friction force attains reliable feeding without slip with respect to the uppermost sheet, and relatively large apparent hardness of the separation roller 230 and the upward convex nip N attains high separation performance for the lower sheets.

The elastic layer 222 of the feed roller 220 and the elastic layer 232 of the separation roller 230 are made of the same rubber, which prevents movement of liquid component in the rubber from the feed roller 220 to the separation roller 230 or from the separation roller 230 to the feed roller 220 and occurrence of the swelling.

The liquid component of the rubber oozes out from the compounding agent such as vulcanizing agent, vulcanization accelerator, reinforcing agent, antioxidant, filler, plasticizer, colorant and contains mineral oil such as gasoline and lubricating oil, various animal and vegetable oils, solvents, and chemicals. The occurrence of the swelling causes changes in diameters of the feed roller 220 and the separation roller 230 that results in a change in a sheet conveyance amount, which increases paper jam probability. However, in the present embodiment, preventing the occurrence of the swelling by using the same material, that is, the same rubber in the feed roller 220 and the separation roller 230 prevents occurrence of paper jam due to the swelling.

Second Embodiment

FIG. 4A illustrates the assembled feed roller 220 according to the second embodiment. FIG. 4B is an exploded perspective view illustrating the feed roller 220 according to the second embodiment. FIGS. 4C and 4D illustrate the pair of rollers using the feed roller 220 according to the second embodiment.

The feed roller 220 includes a concavo-convex portion formed on the outer peripheral surface of the elastic layer 222. The concavo-convex portion includes convex ridges 222 a and concave streaks 222 b which extend in the axial direction of the feed roller 220 and are alternately disposed at equal intervals in the circumferential direction. The hub 221 is fitted inside the elastic layer 222 from the arrow direction in FIG. 4B. The hub 221 has the same configuration as those used in the first embodiment.

Although the cross-sectional shape of the concavo-convex portion is a rectangular shape in the illustrated example, the concavo-convex portion may have any other shape such as a trapezoidal shape or a triangular shape. The concavo-convex portion may have a grain pattern or a wrinkle pattern which have a depth of, for example, less than 1 mm by die transfer, or a polishing mark pattern by polishing.

The concavo-convex portion formed on the outer peripheral surface of the elastic layer 222 as described above softens the apparent hardness of the elastic layer 222. FIG. 4C illustrates the pair of rollers 210 using the feed roller 220 illustrated in FIG. 4A. The lower separation roller 230 is the same as the one used in the first embodiment.

In FIG. 4C, the apparent hardness of the separation roller 230 can be relatively set harder than the apparent hardness of the feed roller 220 because the feed roller 220 has a concavo-convex portion formed on the outer peripheral surface of the elastic layer 222 of the feed roller 220, and the separation roller 230 does not have the concavo-convex portion formed on the inner and outer peripheral surface of the separation roller 230. Therefore, even when the elastic layer 222 of the feed roller 220 and the elastic layer 232 of the separation roller 230 have the same thickness, the relatively harder apparent hardness of the separation roller 230 can form the nip shape at a pressure contact portion in which both the feed roller 220 and the separation roller 230 press against each other to be concave toward the feed roller 220.

As illustrated in FIG. 4D, even when not only the feed roller 220 but also the separation roller 230 also has the concavo-convex portion including convex ridges 232 a and concave streaks 232 b on the outer peripheral surface of the elastic layer 232, setting a height of the concavo-convex portion on the separation roller 230 that is a height between the top of the convex ridge 232 a and the bottom of the concave streak 232 b lower than a height of the concavo-convex portion on the feed roller 220 that is a height between the top of the convex ridge 222 a and the bottom of the concave streak 222 b on the feed roller 220 enables the apparent hardness of the separation roller 230 to be relatively harder than that of the feed roller 220 like above-described embodiment, which results in the nip shape to be concave toward the feed roller 220.

Third Embodiment

FIG. 5A illustrates the assembled feed roller 220 according to the third embodiment. FIG. 5B is an exploded perspective view illustrating the feed roller 220 according to the third embodiment. FIGS. 5C and 5D illustrate the pair of rollers using the feed roller 220 according to the third embodiment.

The feed roller 220 includes a concavo-convex portion formed on the inner peripheral surface of the elastic layer 222. The concavo-convex portion includes convex ridges 222 c and concave streaks 222 d which extend in the axial direction of the feed roller 220 and are alternately disposed at equal intervals in the circumferential direction. The hub 221 is fitted inside the elastic layer 222 from the arrow direction in FIG. 5B. The hub 221 has the same configuration as those used in the first embodiment.

Although the cross-sectional shape of the concavo-convex portion is a rectangular shape in the illustrated example, the concavo-convex portion may have any other shape such as a trapezoidal shape or a triangular shape. The concavo-convex portion may have a grain pattern or a wrinkle pattern which have a depth of, for example, less than 1 mm by die transfer, or a polishing mark pattern by polishing.

The concavo-convex portion formed on the inner peripheral surface of the elastic layer 222 as described above softens the apparent hardness of the elastic layer 222. FIG. 5C illustrates the pair of rollers 210 using the feed roller 220 illustrated in FIG. 5A. The lower separation roller 230 is the same as the one used in the first embodiment.

In FIG. 5C, the apparent hardness of the separation roller 230 can be relatively set harder than the apparent hardness of the feed roller 220 because the feed roller 220 has a concavo-convex portion formed on the inner peripheral surface of the elastic layer 222 of the feed roller 220, and the separation roller 230 does not have the concavo-convex portion formed on the inner and outer peripheral surface of the elastic layer 222 in the separation roller 230. Therefore, even when the elastic layer 222 of the feed roller 220 and the elastic layer 232 of the separation roller 230 have the same thickness, similar to the above-described embodiment, the relatively harder apparent hardness of the separation roller 230 can form the nip shape at the pressure contact portion in which both the feed roller 220 and the separation roller 230 press against each other to be concave toward the feed roller 220.

As illustrated in FIG. 5D, even when not only the feed roller 220 but also the separation roller 230 also has the concavo-convex portion including convex ridges 232 c and concave streaks 232 d on the inner peripheral surface of the elastic layer 232, setting a height of the concavo-convex portion on the separation roller 230 lower than a height of the concavo-convex portion on the feed roller 220 enables the apparent hardness of the separation roller 230 to be relatively harder than that of the feed roller 220 like above-described embodiment, which results in the nip shape to be concave toward the feed roller 220.

Fourth Embodiment

FIG. 6A illustrates the assembled feed roller 220 according to the fourth embodiment. FIG. 6B is an exploded perspective view illustrating the feed roller 220 according to the fourth embodiment. FIGS. 6C and 6D illustrate the pair of rollers using the feed roller 220 according to the third embodiment.

The feed roller 220 includes a concavo-convex portion formed on the outer peripheral surface of the hub 221. The concavo-convex portion includes convex ridges 221 e and concave streaks 221 f which extend in the axial direction of the feed roller 220 and are alternately disposed at equal intervals in the circumferential direction. The hub 221 is fitted inside the elastic layer 222 from the arrow direction in FIG. 6B.

Although the cross-sectional shape of the concavo-convex portion is a rectangular shape in the illustrated example, the concavo-convex portion may have any other shape such as a trapezoidal shape or a triangular shape. The concavo-convex portion may have a grain pattern or a wrinkle pattern which have a depth of, for example, less than 1 mm by die transfer, or a polishing mark pattern by polishing.

The concavo-convex portion formed on the outer peripheral surface of the hub 221 as described above softens the apparent hardness of the elastic layer 222 that is fitted to the outer peripheral surface of the hub 221. FIG. 6C illustrates the pair of rollers 210 using the feed roller 220 illustrated in FIG. 6A. The lower separation roller 230 is the same as the one used in the first embodiment.

In FIG. 6C, the apparent hardness of the separation roller 230 can be relatively set harder than the apparent hardness of the feed roller 220 because the feed roller 220 has a concavo-convex portion formed on the outer peripheral surface of the hub 221 of the feed roller 220, and the separation roller 230 does not have the concavo-convex portion formed on the outer peripheral surface of the hub 231 in the separation roller 230. That is, the inner peripheral surface of the elastic layer 222 on the feed roller 220 receives pressure which works at the nip N, and a part of the elastic layer 222 expands into the concave streaks 221 f on the outer peripheral surface of the hub 221, but the separation roller 230 not having the concave-convex portion does not have a space into which the elastic layer 232. Therefore, even when the elastic layer 222 of the feed roller 220 and the elastic layer 232 of the separation roller 230 have the same thickness, the relatively harder apparent hardness of the separation roller 230 can form the nip shape at a pressure contact portion in which both the feed roller 220 and the separation roller 230 press against each other to be concave toward the feed roller 220.

As illustrated in FIG. 6D, even when not only the feed roller 220 but also the separation roller 230 also has the concavo-convex portion including convex ridges 231 e and concave streaks 231 f on the outer peripheral surface of the hub 231, setting a height of the concavo-convex portion on the separation roller 230 lower than a height of the concavo-convex portion on the feed roller 220 enables the apparent hardness of the separation roller 230 to be relatively harder than that of the feed roller 220 like above-described embodiment, which results in the nip shape to be concave toward the feed roller 220.

The present disclosure is not limited to the details of the embodiments described above, and various modifications and improvements are possible. For example, the concavo-convex portion may be configured as a plurality of convex ridges and a plurality of concave streaks which are formed on at least one of the inner and outer peripheral surface of the elastic layer on the rollers and the outer peripheral surface of the hub in the circumferential direction.

Further, the concavo-convex portion may be configured as a plurality of projections or depressions formed in a scattered pattern, and in this case, the apparent hardness of the feed roller 220 and the separation roller 230 can be adjusted by changing at least one of the height of the projection, the depth of the depression, formation density of the projections, formation density of depressions, a diameter of the projection, and a diameter of the depression. The apparent hardness of the feed roller 220 and the separation roller 230 may be adjusted by adjusting a set including two or more of the concavo-convex portion, the projections, the depressions, and the elastic layer.

The apparent hardness of the feed roller 220 and the separation roller 230 may be adjusted by forming a foamed elastic layer in the elastic layer of the feed roller. A material of the foamed elastic layer may be, for example, resin foam or rubber foam having a closed cell structure. When the separation roller has the foamed elastic layer, the apparent hardness of the feed roller 220 and the separation roller 230 may be adjusted by setting an expansion ratio of the foamed elastic layer on the feed roller 220 larger than an expansion ratio of the foamed elastic layer on the separation roller.

It is to be noted that the above embodiments are presented as examples to realize the present disclosure and are not intended to limit the scope of the disclosure. These novel embodiments can be implemented in various other forms, and various omissions, substitutions, and changes can be made without departing from the gist of the disclosure. These embodiments and variations are included in the scope and gist of the disclosure and are included in the disclosure described in the claims and the equivalent scope thereof. 

What is claimed is:
 1. A sheet feeder comprising: a stacker to stack sheets; and a separation and conveyance device to separate one sheet from the sheets fed from the stacker and convey the sheets one by one, the separation and conveyance device including: a feed roller that rotates in a direction to feed the sheet; and a separation roller to press against the feed roller via the sheet, the separation roller having an apparent hardness different from an apparent hardness of the feed roller and configured to form an indentation in the feed roller.
 2. The sheet feeder according to claim 1, wherein each of the feed roller and the separation roller includes: a rotational shaft; a cylindrical hub supported on the rotational shaft; and a cylindrical elastic layer fitted to an outer peripheral surface of the hub, and wherein a thickness of the elastic layer of the feed roller is thicker than a thickness of the elastic layer of the separation roller to cause a difference in the apparent hardness between the feed roller and the separation roller.
 3. The sheet feeder according to claim 1, wherein each of the feed roller and the separation roller includes: a rotational shaft; a cylindrical hub supported on the rotational shaft; and a cylindrical elastic layer fitted to an outer peripheral surface of the hub and having a concavo-convex portion on an outer peripheral surface of the elastic layer, and wherein a height of the concavo-convex portion of the feed roller is higher than a height of the concavo-convex portion of the separation roller to cause a difference in the apparent hardness between the feed roller and the separation roller.
 4. The sheet feeder according to claim 1, wherein each of the feed roller and the separation roller includes: a rotational shaft; a cylindrical hub supported on the rotational shaft; and a cylindrical elastic layer fitted to an outer peripheral surface of the hub, and wherein the feed roller has a concavo-convex portion on an outer peripheral surface of the elastic layer to cause a difference in the apparent hardness between the feed roller and the separation roller.
 5. The sheet feeder according to claim 1, wherein each of the feed roller and the separation roller includes: a rotational shaft; a cylindrical hub supported on the rotational shaft; and a cylindrical elastic layer fitted to an outer peripheral surface of the hub and having a concavo-convex portion on an inner peripheral surface of the elastic layer, and wherein a height of the concavo-convex portion of the feed roller is higher than a height of the concavo-convex portion of the separation roller to cause a difference in the apparent hardness between the feed roller and the separation roller.
 6. The sheet feeder according to claim 1, wherein each of the feed roller and the separation roller includes: a rotational shaft; a cylindrical hub supported on the rotational shaft; and a cylindrical elastic layer fitted to an outer peripheral surface of the hub, and wherein the feed roller has a concavo-convex portion on an inner peripheral surface of the elastic layer to cause a difference in the apparent hardness between the feed roller and the separation roller.
 7. The sheet feeder according to claim 1, wherein each of the feed roller and the separation roller includes: a rotational shaft; a cylindrical hub supported on the rotational shaft; and having a concavo-convex portion on an outer peripheral surface of the hub, and a cylindrical elastic layer fitted to the outer peripheral surface of the hub, and wherein a height of the concavo-convex portion of the feed roller is higher than a height of the concavo-convex portion of the separation roller to cause a difference in the apparent hardness between the feed roller and the separation roller.
 8. The sheet feeder according to claim 1, wherein each of the feed roller and the separation roller includes: a rotational shaft; a cylindrical hub supported on the rotational shaft; and a cylindrical elastic layer fitted to an outer peripheral surface of the hub, and wherein the feed roller has a concavo-convex portion on the outer peripheral surface of the hub to cause a difference in the apparent hardness between the feed roller and the separation roller.
 9. The sheet feeder according to claim 1, wherein each of the feed roller and the separation roller includes: a rotational shaft; a cylindrical hub supported on the rotational shaft; and a cylindrical elastic layer fitted to an outer peripheral surface of the hub, and wherein the elastic layer of the feed roller is a foamed elastic layer to cause a difference in the apparent hardness between the feed roller and the separation roller.
 10. The sheet feeder according to claim 1, wherein each of the feed roller and the separation roller includes: a rotational shaft; a cylindrical hub supported on the rotational shaft; and a cylindrical foamed elastic layer fitted to an outer peripheral surface of the hub, and wherein an expansion ratio of the foamed elastic layer of the feed roller is larger than an expansion ratio of the foamed elastic layer of the separation roller to cause a difference in the apparent hardness between the feed roller and the separation roller.
 11. The sheet feeder according to claim 1, wherein each of the feed roller and the separation roller includes: a rotational shaft; a cylindrical hub supported on the rotational shaft; and a cylindrical elastic layer fitted to an outer peripheral surface of the hub, and wherein at least one of the feed roller and the separation roller has a concavo-convex portion on at least one of the hub and the elastic layer, and the concavo-convex portion has a plurality of convex ridges and a plurality of concave streaks that extend in a direction of the rotational shaft of the at least one of the feed roller and the separation roller.
 12. The sheet feeder according to claim 1, wherein each of the feed roller and the separation roller includes: a rotational shaft; a cylindrical hub supported on the rotational shaft; and a cylindrical elastic layer fitted to an outer peripheral surface of the hub, and wherein at least one of the feed roller and the separation roller has a concavo-convex portion on at least one of the hub and the elastic layer, and the concavo-convex portion has a plurality of convex ridges and a plurality of concave streaks that extend in a circumferential direction of the at least one of the feed roller and the separation roller.
 13. The sheet feeder according to claim 1, wherein each of the feed roller and the separation roller includes: a rotational shaft; a cylindrical hub supported on the rotational shaft; and a cylindrical elastic layer fitted to an outer peripheral surface of the hub, and wherein at least one of the feed roller and the separation roller has a concavo-convex portion on at least one of the hub and the elastic layer, and the concavo-convex portion has a plurality of projections formed in a scattered pattern.
 14. The sheet feeder according to claim 1, wherein each of the feed roller and the separation roller includes: a rotational shaft; a cylindrical hub supported on a rotational shaft; and a cylindrical elastic layer fitted to an outer peripheral surface of the hub, and wherein at least one of the feed roller and the separation roller has a concavo-convex portion on at least one of the hub and the elastic layer, and the concavo-convex portion has a plurality of depressions formed in a scattered pattern.
 15. The sheet feeder according to claim 1, wherein the feed roller and the separation roller are made of a same material.
 16. The sheet feeder according to claim 1, wherein the feed roller and the separation roller have a same diameter.
 17. An image forming apparatus comprising: the sheet feeder according to claim 1, and an image forming device to form an image on the sheet sent from the sheet feeder.
 18. A sheet feeder comprising: a stacker to stack sheets; and a separation and conveyance device to separate one sheet from the sheets fed from the stacker and convey the sheets one by one, the separation and conveyance device including: a feed roller that rotates in a direction to feed the sheet; and a separation roller to press against the feed roller via the sheet, the separation roller having an apparent hardness harder than an apparent hardness of the feed roller. 